1. Field of the Invention
The present invention generally relates to electro-optical readers, such as scanners and imagers, for reading codes such as bar code symbols and, more particularly, to an optical bandpass filter for filtering light traveling along a path to a photodetector in such readers, and especially to rendering the filter of non-heavy metals.
2. Description of the Related Art
Electro-optical readers, such as bar code symbol scanners and solid state imagers, have found wide acceptance in retail, wholesale, industrial and military applications. The scanner typically illuminates a symbol comprised of regions of different light reflectivity, senses light of different intensity scattered from the symbol regions with a photodetector such as a photodiode, and determines widths and spacings of the symbol regions to derive information encoded in the symbol. The imager determines the widths and spacings of the symbol regions by capturing and processing an image of the symbol, the image capture being performed by a photodetector such as an array of photocells.
In both types of readers, the photodetector senses the light incident thereon. The light includes the desired light scattered from the symbol regions, as well as undesired light, such as ambient light, sunlight, and light reflected off other objects in the field of view of the reader. An optical bandpass filter is commonly employed in front of the photodetector to allow only the desired light to pass through to the photodetector, and to reject the undesired light. Detection of the undesired light compromises reader performance and can even lead to failure to read the symbol.
It was known in the prior art to make an optical bandpass filter using a clear glass member with an infrared-blocking coating and a Wratten filter, which consists of a brittle cellophane material. It was expensive and difficult to cut to shape the Wratten filter without waste since the Wratten filter tended to fall apart during cutting or punching.
In the case of a moving beam reader where a red laser beam having a wavelength of about 650 nm is swept across the symbol to be read, it was known in the prior art to make the exit window of the reader of a red-colored, plastic material, such that the window itself formed part of the bandpass filter. However, the use of plastic for the exit window made the reader susceptible to scratching and color fading due to exposure to sunlight or chemicals.
It was also known to position the optical bandpass filter inside the reader in front of the photodetector. In the case of the red laser beam, the filter included a red-colored glass member having a dielectric coating to set the higher passband value. The red color of the glass member is caused by a coloring, one of whose components is cadmium which is operative to set the lower passband value. The cadmium is introduced while the glass member is in a molten state.
The use of cadmium, however, is considered by some governmental authorities to pose a potential hazard to the environment. Cadmium is a known eye and skin irritant and has been linked to lung and kidney disease, as well as cancer. Cadmium, like other heavy metals, can be a source of pollution if it is present in high concentrations, or if it is extracted from the glass member by either some natural process, or during the disposal process, for example, by grinding or pulverization.